The Santa Ana winds sweep down from the deserts and across coastal southern California, pushing dust and smoke from wildfires far out over the Pacific Ocean. Los Angeles is in the upper left of this image, and San Diego is near the center.

The Santa Ana winds are strong, extremely dry down-slope winds that originate inland and affect coastal Southern California and northern Baja California. Santa Ana winds blow mostly in autumn and winter, but can arise at other times of the year also. The winds originate from cool, dry high-pressure air masses in the Great Basin. The winds are known especially for the hot dry weather (often the hottest of the year) that they bring in the fall, and are infamous for fanning regional wildfires. For these reasons, they are sometimes known as the "devil winds" across Southern California.[1][2]

The National Weather Service defines Santa Ana winds as "Strong down slope winds that blow through the mountain passes in southern California. These winds, which can easily exceed 40 miles per hour (18 m/s), are warm and dry and can severely exacerbate brush or forest fires, especially under drought conditions."[3]

This map illustration shows a characteristic high-pressure area centered over the Great Basin, with the clockwise anticyclone wind flow out of the high-pressure center giving rise to a Santa Ana wind event as the airmass flows through the passes and canyons of southern California, manifesting as a dry northeasterly wind.

The Santa Anas are a katabatic wind—katabatic meaning "to flow downhill" in Greek, which is an accurate description of the action of these winds.[4] Santa Ana winds originate from high-pressure airmasses over the Great Basin and upper Mojave Desert. Any low-pressure area over the Pacific Ocean, off the coast of California, can change the stability of the Great Basin High, causing a pressure gradient that turns the synoptic scale winds southward down the eastern side of the Sierra Nevada and into the Southern California region.[5] Cool, dry air flows outward in a clockwise spiral from the high pressure center. This cool, dry airmass sweeps across the deserts of eastern California toward the coast, and encounters the towering Transverse Ranges, which separate coastal Southern California from the deserts. The airmass, flowing from high pressure in the Great Basin to a low pressure center off the coast, takes the path of least resistance by channeling through the mountain passes to the lower coastal elevations, as the low pressure area off the coast pulls the airmass offshore.

These passes include the Soledad Pass, the Cajon Pass, and the San Gorgonio Pass, each of which is well known for its connection to the Santa Ana winds.[6] As the air is funneled into the mountain passes, its velocity increases dramatically, often to near-gale force or above. At the same time, as the air descends from higher elevation to lower, it is heated adiabatically, warming about 5 °F for each 1,000 feet it descends (10 °C for each 1,000 m).[6] As it warms, its capacity to hold moisture increases, so its relative humidity decreases. The air has already been dried by orographic lift before reaching the Great Basin, as well as by subsidence from the upper atmosphere, so this additional warming often causes relative humidity to fall below 10 percent.[7] The end result is a strong, warm, and very dry wind blowing out of the bottom of mountain passes into the valleys and coastal plain. The warm Santa Ana winds cause the Southern California coastal region to reach some of its annual high temperatures in autumn rather than summer. During Santa Ana conditions it is typically hotter along the coast than in the deserts.[8]

It is often said that the air is heated and dried as it passes through the Mojave and Sonorandeserts, but according to meteorologists this is a popular misconception. The Santa Ana winds usually form during autumn and early spring, when the surface air in the elevated regions of the Great Basin and Mojave Desert (the "high desert") is cool or even cold.

Note that while the Santa Ana Winds are a katabatic wind, they are not a Föhn wind. A Föhn wind results from precipitation on the windward side of a mountain range which releases latent heat into the atmosphere which is then warmer on the leeward side (e.g., the Chinook or the original Föhn). The Santa Ana winds do not originate in precipitation, but cool, dry, and stable high-pressure airmasses in the high deserts.

If the Santa Anas are strong, the usual day-time sea breeze may not arise, or develop weak later in the day because the strong offshore desert winds oppose the on-shore sea breeze. At night, the Santa Ana Winds merge with the land breeze and strengthen because the inland desert cools more than the ocean due to differences in the heat capacity and because there is no competing sea breeze.[7][9]

Santa Ana winds often bring the lowest relative humidities of the year to coastal Southern California. These low humidities, combined with the warm, compressionally-heated airmass, plus the high wind speeds, create critical fire weather conditions. The combination of wind, heat, and dryness accompanying the Santa Ana winds turns the chaparral into explosive fuel feeding the infamous wildfires for which the region is known. Wildfires fanned by Santa Ana winds burned 721,791 acres (2,920.98 km2) in two weeks during October 2003.[10] These same winds have contributed to the October 2007 California wildfires that burned over 500,000 acres (2,000 km2).[11]

Although the winds often have a destructive nature, they have some benefits as well. They cause cold water to rise from below the surface layer of the ocean, bringing with it many nutrients that ultimately benefit local fisheries. As the winds blow over the ocean, sea surface temperatures drop about 4°C (7°F), indicating the upwelling. Chlorophyll concentrations in the surface water go from negligible, in the absence of winds, to very active at more than 1.5 milligrams per cubic meter in the presence of the winds.

During the Santa Ana winds, large ocean waves can develop. These waves come from a northeasterly direction; toward the normally sheltered side of Catalina Island. Protected harbors such as Avalon and Two Harbors are normally sheltered and the waters within the harbors are very calm. In strong Santa Ana conditions, these harbors develop high surf and strong winds that can tear boats from their moorings and crash them onto the shore. During a Santa Ana, it is advised that boaters moor on the back side of the island to avoid the dangerous conditions of the front side.

A Santa Ana fog is a derivative phenomenon in which a ground fog settles in coastal Southern California at the end of a Santa Ana wind episode. When Santa Ana conditions prevail, with winds in the lower two to three kilometers (1.25-1.8 miles) of the atmosphere from the north through east, the air over the coastal basin is extremely dry, and this dry air extends out over offshore waters of the Pacific Ocean. When the Santa Ana winds cease, the cool and moist marine layer may re-form rapidly over the ocean if conditions are right. The air in the marine layer becomes very moist and very low clouds or fog occurs.[12][13] If wind gradients turn on-shore with enough strength, this sea fog is blown onto the coastal areas. This marks a sudden and surprising transition from the hot, dry Santa Ana conditions to cool, moist, and gray marine weather, as the Santa Ana fog can blow onshore and envelope cities in as quickly as fifteen minutes. However, a true Santa Ana fog is rare, because it requires conditions conducive to rapid re-forming of the marine layer, plus a rapid and strong reversal in wind gradients from off-shore to on-shore winds. More often, the high pressure system over the Great Basin, which caused the Santa Ana conditions in the first place, is slow to weaken or move east across the United States. In this more usual case, the Santa Ana winds cease, but warm, dry conditions under a stationary air mass continue for days or even weeks after the Santa Ana wind event ends.

A related phenomenon occurs when the Santa Ana condition is present but weak, allowing hot dry air to accumulate in the inland valleys that may not push all the way to sea level. Under these conditions auto commuters can drive from the San Fernando Valley where conditions are sunny and warm, over the low Santa Monica Mountains, to plunge into the cool cloudy air, low clouds, and fog characteristic of the marine air mass. This and the "Santa Ana fog" above constitute examples of an air inversion.

The similar winds in the Santa Barbara area occur most frequently in the late spring to early summer, and are strongest at sunset, or "sundown"; hence their name: sundowner. Because high pressure areas usually migrate east, changing the pressure gradient in southern California to the northeast, it is common for "sundowner" wind events to precede Santa Ana events by a day or two.[14]

Winds blowing off the elevated glaciated plateaus of Greenland and Antarctica experience the most extreme form of katabatic wind, of which the Santa Ana is a type, for the most part. The winds start at a high elevation and flow outward and downslope, attaining hurricane gusts in valleys, along the shore, and even out to sea. Like the Santa Ana, these winds also heat up by compression and lose humidity, but because they start out so extraordinarily cold and dry and blow over snow and ice all the way to the sea, the perceived similarity is negligible.

This section needs expansion. You can help by adding to it.(December 2011)

The Santa Ana winds and the accompanying raging wildfires have been a part of the ecosystem of the Los Angeles Basin for over 5,000 years, dating back to the earliest habitation of the region by the Tongva and Tataviam peoples.[15]

The Santa Ana winds have been recognized and reported in English-language records as a weather phenomenon in Southern California since at least the mid-nineteenth century.[1] Various episodes of hot, dry winds have been described over this history as dust storms, hurricane-force winds, and violent north-easters, damaging houses and destroying fruit orchards. Newspaper archives have many photographs of regional damage dating back to the beginnings of news reporting in Los Angeles. When the Los Angeles Basin was primarily an agricultural region, the winds were feared particularly by farmers for their potential to destroy crops.[1]

In early December 2011, the Santa Ana winds were the strongest yet recorded. At atmospheric set-up occurred that allowed the towns of Pasadena and Altadena in the San Gabriel Valley to get whipped by 110 MPH sustained winds and 170 MPH gusts (unconfirmed).[16] The winds toppled thousands of trees, knocking out power for over a week. Schools were closed, and a "state of emergency" was declared. The winds grounded planes at LAX, destroyed homes and were even strong enough to snap a concrete stop light from its foundation.[17] The winds also ripped through Mammoth Mountain and parts of Utah. Mammoth Mountain hold California highest wind gust of 175 MPH from that night.

Especially hot, dry, and dusty Santa Ana winds are widely believed (in Southern California, at least) to affect people's moods and behavior negatively. This has not been definitively proven in studies, although limited evidence may point to this conclusion.[18] Even without ironclad scientific proof, it is a well-accepted part of local lore.[2]

There is some belief the winds also create positive ions, which are believed to affect mood negatively. Many believe this to be the cause for the statistical increase in the number of suicides and homicides during these times.[2][23][24][25][26][27]

The most well-accepted explanation for the name Santa Ana winds is that it is derived from the Santa Ana Canyon in Orange County, one of the many locations the winds blow intensely.[1][4] Newspaper references to the name Santa Ana winds date as far back as 1886. By 1893, controversy had broken out over whether this name was a corruption of the Spanish term Santana (a running together of the words Santa Ana), or the different term Satanás, meaning satan. However, newspaper mention of the term "Satanás" in reference to the winds did not begin appearing until more than 60 years later. A possible explanation is that the Spanish language merges two identical vowels in elision. Thus the Spanish pronunciation of the phrase "Santa Ana" sounds like "Santana".[citation needed]

Another attempt at explanation of the name claims that it derives from a Native American term for "devil wind" that was altered by the Spanish into the form "Satanás" (meaning Satan), and then later corrupted into "Santa Ana." However, an authority on Native American language claims this term "Santana" never existed in that tongue.[4]

A third explanation places the origin of the term Santa Ana winds with an Associated Press correspondent stationed in Santa Ana in 1902, who documented the name "Santa Ana winds," or possibly, mistook the term "Satana" for "Santa Ana." [28]

There was a desert wind blowing that night. It was one of those hot dry Santa Anas that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks. Anything can happen. You can even get a full glass of beer at a cocktail lounge.

The baby frets. The maid sulks. I rekindle a waning argument with the telephone company, then cut my losses and lie down, given over to whatever is in the air. To live with the Santa Ana is to accept, consciously or unconsciously, a deeply mechanistic view of human behavior.

...[T]he violence and the unpredictability of the Santa Ana affect the entire quality of life in Los Angeles, accentuate its impermanence, its unreliability. The wind shows us how close to the edge we are.